Cast moulding is a known method for the production of contact lenses. It offers considerable quality and cost advantages over other methods such as lathing and/or spin-casting. It is a highly repeatable process.
During cast moulding it is important to maintain a good seal between the mould pieces in order to prevent the loss of volatile components from monomer used to make the lens and to provide a clean edge profile to the cast lens. Using existing methods the mould pieces are generally held together by mechanical means. However, during curing the mould pieces may relax resulting in uneven sealing loads. It is also difficult to design moulds where sufficient pressure is applied to the mould parts at the seal line between the mould parts so as to separate the monomer in the lens cavity from any excess monomer which is displaced into flash cavities. Imperfectly made seals result in damaged lenses and costly inspection is needed to separate reject from good lenses.
During cast moulding it is also important to compensate for monomer shrinkage which occurs during the polymerising curing process in order to prevent the formation of voids or bubbles in the lens due to cavitation effect. To reduce this problem existing methods are known which provide one of the mould pieces with a flexible rim which collapses during polymerisation. Other systems allow monomer from a reservoir cavity in the mould to flow back into the lens cavity due to suction effect of shrinkage and other systems allow the mould surfaces to move towards one another under suction effect. However, the flexible rim and reservoir methods disturb the seal and reject lenses or lenses requiring edge polishing result. An advantage of mould surface movement over other methods is that it does allow the seal line to be maintained intact but the mould shells must be carefully designed to collapse evenly under suction forces. Variations in mould thickness, material or cure temperature will affect mould stiffness and hence the rate and amount of collapse. The monomer shrinkage induced suction has to overcome the natural stiffness of the mould structure without for example sucking monomer in from a flash cavity around the mould. During cast moulding it is necessary to provide a cavity or cavities within the mould assembly to contain excess monomer squeezed out from the lens cavity as the mould pieces are brought together. This excess monomer forms flash during the curing process and this flash must be separated from the lens prior to packing. In some cases the flash is preferentially fixed to one of the mould pieces by the provision of snatch rings but generally the separation process is carried out manually and is costly.
During cast moulding the lens will, after curing, tend to adhere to one or other of the mould pieces in a random manner unless the moulds are designed to hold the lens on one specific half. There are processing advantages in being able to control or define on which mould half the lens will be held when the mould pieces are separated after curing. Moulds which rely on flexible rims to compensate for monomer shrinkage will tend to retain the lens on the mould piece which has the rim. This is because the rim deflects generally into the lens cavity where the monomer hardens around it. While this process can, to some extent, be relied upon to fix the lens on one half, the net effect of the rim is to deform the edge of the lens resulting in inconsistent edge profiles, reject lenses and edge polishing even of the acceptable lenses i.e. those which have not been damaged by mechanical removal from the retaining mould half. Another method for retaining the lens on one specific mould piece is to create a return on the edge of the lens cavity in such a way as to wedge the hard lens. For example a lens cavity on the concave mould may have a returned edge profile which will retain the hard lens. Unfortunately this creates considerable constraints on the design of the lens edge profile and the creation of relatively thick uncomfortable lens edges.
During the process of cast moulding certain batches may require to be tinted or coloured. There are a number of tinting systems but they require the careful location and support of the lens such that ink or dye can be precisely applied to the lens surface generally the front convex surface without damage to the lens. This positioning of the lens is a labour intensive operation requiring considerable operator skill if rejects are to be minimised.
During the process of cast moulding it may be desirable to carry out some form of work on the lens such as surface treatment. There are cost and quality advantages in carrying out these operations while the lens is still captive on one of the mould pieces but it is necessary to be able to guarantee on which mould half the lens is retained.
During the process of cast moulding the lens is handled in a number of different containers as it undergoes inspection, power measurement, extraction, hydration and transfer to the final pack, generally a glass vial. There can be cost and quality advantage if these operations can be carried out in the mould which is also a main component in the final pack. There are also advantages if the mould piece forming part of the pack is of such design that the lens is properly orientated and accessible for ease of removal by the user. This may be achieved by offering the lens with the convex surface uppermost. Alternatively there may be advantage in providing a "basket" or carrier device within the pack which contains the lens and which is accessible to the user thereby making lens removal from the pack easy.
During cast moulding it is extremely important to protect the optical surfaces of the mould pieces since the slightest damage mark will be transferred to the lens resulting in a reject. Damage can occur for example when the mould is ejected from the moulding machine. Similarly for hygiene reasons it is important to minimise the possibility of an optical surface coming into contact even with soft objects.
It is an object of the present invention to provide improved apparatus and methods for cast moulding contact lenses. It is also an object to provide an improved contact lens and an improved packaged contact lens.
It is a further object of the present invention to provide an improved method and apparatus in which the cast lens is supported on the male mould surface when opened and an effective package is formed by sealing the lens in a container formed by part of the mould.